Of course current drive is hardly applicable to speakers which have been designed for voltage drive, i.e. low output resistance.
But it it definetly not a nonsense to build a speaker with current drive in mind, especially for diy or as an active speaker.

High output impedance of power amplifiers, as seen in many tube amps, IS a bad thing. It will create frequency response with dips and bumps related to speaker impedance curve, and change Q of the bass driver.

Exactly, other than the "bad thing." Whether or not it's a "bad thing" depends on the speaker. See Morgan Jones's "Arpeggio", for example, for how the high source impedance of a SET amp can be exploited to make a flatter overall frequency response. And don't forget the extensive work that Nelson Pass has done with single driver systems and current drive amps.

High output impedance of power amplifiers, as seen in many tube amps, IS a bad thing.

REALLY? I mean it is REALLY "a bad thing"?

I mean, it gives you cancer or something (okay, I'm kidding).

Allow me to take a concrete example. Here is the measured frequency response of a speaker (taken from Stereophile), measured with essentially zero source impedance:

Now let us view the speakers impedance directly below this, shall we?

If we look at the first curve the speaker appears severely non-flat.

Now let's look at the impedance response and estimate what would happen with the kind of source impedance the Cary Amplifier you showed as example of just HOW BAD tube amp's are (say 4 Ohm or a DF of 2)?

First, the peak at 10KHz will be reduced by 1.5dB,.
Second, the region bewteen 600Hz and 4KHz is boosted by as much as 3dB (around 1.5KHz), less towards the limts.
Third, the boosted region around 250Hz will be attenuated by around 3dB.
Fourth, the frequency region around 50Hz is boosted by around 6dB.

In fact, if we where driving this speaker from such a source impedance we would find it to have a reasonably flat frequency response (on axis), contrary to the picture as it appears, because, at least in THIS CASE, the extra source impedance is needed to make the speaker perform correctly. And of course, this speaker was designed to partner Tube Amplifiers with a significnant output impedance.

Quote:

Originally Posted by PMA

It will create frequency response with dips and bumps related to speaker impedance curve, and change Q of the bass driver. Sometimes it works like a sort of tone controls. Some may like it, I understand, but it has nothing to do with sound fidelity.

Pavel, your dynamic speakers are kind of artificial. We speak here about experience with real amp+speaker systems. Dynamic speakers are very complex mechanical systems, and electrically can be damped only the main resonant peak on low frequency.

Pavel, your dynamic speakers are kind of artificial. We speak here about experience with real amp+speaker systems. Dynamic speakers are very complex mechanical systems, and electrically can be damped only the main resonant peak on low frequency.

There is no disagreement. That's what I have shown and I have also shown how the added series resistance increases frequency response resonant bump of a single driver, in case that that single driver was not inherently overdamped. And yes we speak about real speakers which have quite uneven impedance vs. frequency plot with peaks and dips. Again any added series resistance worsens frequency response.

In case that you say that the resistor added in series with speaker does not matter, and that tube amplifier large output impedance does not matter, than we disagree.